Liquid level sensor

ABSTRACT

A liquid level sensor having a vertical guide tube with one or more magnetically operated switches therein at vertically spaced locations and a free float thereon which rises and falls with the liquid level and as it passes each switch magnetically latches it in one condition until the float returns in the opposite direction and unlatches it. The switches may be normally open, normally closed, or any combination, so that movement of the float past the switches may provide any desired circuit sequence.

This is a division, of application Ser. No. 627,518, filed Oct. 31,1975.

FIELD OF INVENTION

This invention relates to position sensing apparatus, in particularliquid level sensing devices of the type having a vertical guide tubecontaining magnetic proximity switches and a free float slidable on thetube and buoyant in the liquid to rise and fall with its level, and inpassing the switches actuating them.

BACKGROUND OF THE INVENTION

It is known in the art to provide a liquid level sensor comprising anon-magnetic guide tube vertically arranged in the liquid whose level isto be sensed, with a plurality of reed switches disposed spaced apart inthe tube to be actuated by a magnet carried by a float guided on thetube and rising and falling with the liquid level. The reed switches areconnected in electric circuits to signal the liquid level, and may serveto cause energization of pumps, alarms, indicators and the like. Thefollowing United States patents are representative of such teaching:U.S. Pat. Nos. 3,200,645; 3,788,340; 3,646,293; 3,678,750; 3,685,357.

A problem common to each of these is that while the reed switch isclosed as the float magnet passes, as for example during a rise of thefloat, it then opens as soon as the float magnet passes above theswitch. Consequently, the only time a circuit is completed through thereed switch is when the float magnet field is sweeping it, and in orderto be useful the reed switch must therefor be electrically connected toa system which is undisturbed by the switch opening as the floatcontinues to rise. Such a system may utilize a latching relay as in U.S.Pat. No. 3,685,357 which will maintain a circuit energized even thoughthe reed switch opens. Redundancy problems are raised by sucharrangements to insure that when the reed switch opens it is not becausethe float has reversed direction but is actually still rising. Inaddition, should a float be rising and before reaching a reed switchthere be an electric power failure during the time the float passes theswitch, the circuit will not indicate the increased liquid level whenthe power returns.

To avoid redundancy problems, power failure problems, latching relaysand the like, efforts have been directed to devising means formaintaining the reed switch closed when the float magnet reaches it anddespite the liquid level continuing to rise. One commercially availablesolution, and another shown in U.S. Pat. No. 3,826,139, involves the useof a guide tube having a plurality of floats, one for each reed switch,and stops on the guide tube which limit the rise of each float so thatits ascent is arrested when its magnet has closed the reed switch. Thefloat then remains in this position as the liquid continues to rise thusholding the reed switch closed. The switch is opened when the liquidlevel falls sufficiently to allow the float to drop away from the stopand carry its magnet sufficiently below the reed switch so it can open.In addition to the necessity of using a number of floats, whichincreases the cost of the system, the only way the sensing levels can bechanged is by physically shifting the float stops on the guide tube andthis entails gaining physical access to the outside of the guide tubewhich is sometimes difficult or inconvenient.

Another solution is proposed by U.S. Pat. No. 3,437,771 where a two-partfloat is shown, one part carrying a magnet is intended to lock onto abias magnet at the switch and open the switch and remain at the switchuntil the other float part drops sufficiently to carry the inner floatdown away from the switch and allow the switch to re-close. Thisteaching also uses a stop to limit rise of the float, and would requiremultiple floats and stops if more than two switches (levels) wereinvolved. In addition, the switch is normally closed by the bias magnetand is opened by the float magnet, and the teaching would appear to belimited to this mode of operation.

SUMMARY OF THE INVENTION

We have overcome these objections to the prior art and in addition haveobtained other positive advantages by providing a liquid level sensor inwhich, when the float passes a switch in the guide tube, the switch ismagnetically actuated to a different contact condition which is thenmaintained despite ongoing movement of the float completely past theswitch, until the float returns and passes the switch in the oppositedirection whereupon it will magnetically actuate the switch to returnthis contacts to their initial condition. As a result of this it ispossible to provide a guide tube with a plurality of switches arrangedat various levels, and a single free float which rises and falls withthe liquid level, and which will successively actuate and latch eachswitch it passes as it rises, for example, and then successively unlatcheach switch it passes as it falls. Thus, the need for latching relays inthe switch circuits is eliminated. Actuation and latching of theswitches is accomplished utilizing permanent magnets and is onlydependent upon movement of the float in a given direction past a switch.Accordingly, temporary power failures during movement of the float pasta switch will not affect the logic of our system, and it will not losestep or phase with the liquid level.

Our sensor may also be constructed to provide switches of differentoperating modes at various levels within the same guide tube, viz., oneswitch may be normally open and another normally closed and as the floattravels in one direction along the tube each will have its contactsshifted to the opposite condition.

The switches may be supported in the guide tube for re-positioning froman end of the tube, and in view of the freedom of the float to traversethe tube and latch each switch as it passes, such re-positioning of theswitches allows for a simple adjustment of the liquid level sensingheight without the necessity of gaining access to the float. In otherwords it is not necessary to re-position stops on the outside of theguide tube to limit movement of the float.

In carrying out the invention, magnetic proximity switches such as ofthe reed type are suspended in the guide tube for actuation and responseto the sweep of a magnetic field as the float rises and falls with theliquid level past the switches. Unlike the prior art liquid levelsensors utilizing reed switches, the reed switches of this invention arelatched open or closed, as desired. We have shown various ways ofaccomplishing this; each requiring a certain combination and arrangementof switch and float structures.

According to one approach, a conventional reed switch having normallyopen contacts, is provided with a small bias magnet of a strengthinsufficient alone to close the contacts, but once closed sufficient tohold them closed. Magnet means carried by the float are so arranged asto provide leading and trailing magnetic fields of opposite directionviz., leading and trailing in relation to float movement along the guidetube, and "opposite direction" having reference to the direction of themagnetic lines of flux. When the direction of the trailing field of thefloat magnet sweeping the reed switch augments or compliments thepolarity of the bias magnet, the reed switch is thereupon closed andremains closed under the influence of the bias magnet though the floatmoves on away. The switch will remain closed until the float magnetagain approaches from the opposite direction and the direction of itstrailing field opposes the polarity of the bias magnet, whereupon theswitch will open and remain open though the float continues on past theswitch and moves away. We have shown two approaches to obtaining leadingand trailing fields of opposite direction. According to one approach thefloat magnet means is arranged to have one pole facing radially inwardlytoward the guide tube and the opposite pole facing radially outwardly.According to another approach the float magnet means comprises a pair ofmagnet arrays with each array comprising a plurality of magnets arrangedcircumaxially around the guide tube with the axes of the magnetsextending along the guide tube and with the arrays arrangedlongitudinally along the float in adjacent relation and with commonmagnet poles in confronting relation.

In another embodiment of the invention reed switches having reeds of amaterial capable of having a high residual magnetism (hereinafterreferred to as "self-latching reed switches") are used in the guidetube. Float magnet means are provided which create leading and trailingfields of opposite directions and different strengths which sweep theguide tube and switches as the float rises and falls. When the strongerfield is the trailing field as the float passes over a switch it willclose it and the residual magnetism induced in the switch will hold itclosed. When the float movement reverses and the trailing field is nowthe weaker field and of opposite direction, when such field sweeps theswitch the contacts will open and remain open. Two approaches are shownto obtain magnetic fields of opposite direction and different strengths.

Also shown is an arrangement utilizing a self-latching reed switchwherein the float magnet is similar to that first described, viz.,creates leading and trailing fields of equal but opposite direction.

A novel arrangement for suspending and encapsulating the reed switchassemblies is also disclosed which permits ready adjustment of theliquid level sensing points.

A number of advantages are obtainable from a liquid level sensor of thekind herein disclosed namely:

1. The sensor utilizes only one float to operate any number of switchesinstead of one float for each switch.

2. The sensor uses only one float stop and such is located at the bottomend of the tube.

3. Because the switch action is not dependent upon float stop locations,operating levels are easily adjusted by merely moving the switchlocations up or down in the guide tube as desired.

4. The vertical distance between switches is not restricted because onefloat is used to operate all switches.

5. The switches can be made all normally open, all normally closed orany desired combination of normally open and normally closed.

6. The sensor can be used in any liquid, conductive or non-conductive.

7. We have found that the guide tube may be as small as 1/2 inch OD oras large as 2 inches OD and the float can be as small as 2 inches OD oras large as 14 inches OD and guide tubes and floats of both greater orsmaller dimensions are feasible. Small floats are more suitable forclean low viscosity liquids and large floats for use with high viscosityliquids or heavy sludges.

8. The sensor is suitable for pressurized applications as well asnon-pressurized environments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary side elevation, in cross section, through a tankor other vessel showing a liquid sensor embodying the invention;

FIG. 2 is a fragmentary cross-sectional view of the guide tube and floatof FIG. 1 showing the parts in greater detail;

FIGS. 2A, 2B, 2C and 2D schematically illustrate the magnetic fields andtheir co-action and effect on a reed switch of FIG. 2 during a rise anda fall of the float past the switch;

FIG. 3 is similar to FIG. 2 but wherein the bias magnets are reversed togive the opposite switch action during the same travel of the float;

FIGS. 3A, 3B, 3C and 3D are similar to FIGS. 2A-2D except theyillustrate the action on a reed switch of FIG. 3;

FIG. 4 shows a modified form of magnetic proximity switch and floatmagnet means;

FIGS. 4A, 4B, 4C, 4D and 4E schematically show the force fields of thefloat magnet and the effect on the reed switch of FIG. 4 during a riseand a fall of the float past the switch;

FIG. 5 is a fragmentary cross-sectional view through another form ofproximity switch in the guide tube with a float similar to FIGS. 1-3;

FIGS. 5A, 5B, 5C and 5D illustrate schematically the force fields of thefloat magnet and their effect on a reed switch of FIG. 5 as the floatrises and falls past the switch;

FIG. 6 is a fragmentary cross-sectional view through a guide tube andfloat showing a further modification of the invention;

FIG. 7 is a fragmentary cross-sectional view through a guide tube andfloat embodying a further modification of the invention;

FIG. 8 is a cross-sectional view through a guide tube and floatembodying another form of the invention;

FIGS. 9 and 10 are respectively side and front views showing a preferredarrangement for connecting, protecting and suspending a proximity switchfor insertion in the guide tube;

FIG. 11 shows an embodiment of the float magnet means, portions of thefloat being removed for clarity, and

FIG. 12 is a cross-sectional view taken on the line 12--12 of FIG. 2.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1 an installation for a liquid level sensor has been depicted ascomprising a tank or other vessel 20 having top, side and bottom walls22, 24 and 26 with a liquid 28 therein whose level 30 is to be sensed. Aguide tube 32 is shown arranged vertically within the tank 20 with afree float 34 mounted on the tube for guided movement therealong as theliquid level 30 rises and falls. The buoyancy of the float 34 is suchthat it will be supported by the liquid with approximately 50 percent ofthe float submerged and its "water line" or liquid line at the liquidsurface 30 encircling the float substantially midway between its upperand lower ends.

While the guide tube may be supported in the tank in various ways, inthe embodiment shown in FIG. 1, which is intended to be illustrativeonly, the top wall 22 of the tank contains a pipe threaded opening 36into which is screw threaded a junction box 38 provided with atransverse web 40 within which is threadedly sealed a tube fitting 42telescopingly receiving the guide tube 32 in a slip fit. The upper endof the tube fitting 42 is provided with an internal chamfer and aferrule 44 has a lower end received in the chamfer and the other endtrapped within the lip of a nut 46 threaded on the upper end of the tubefitting, such that as the nut is tightened down the ferrule is squeezedagainst the tube to grip the guide tube and hold it securely in the tubefitting and seal it therein.

The guide tube 32 should be formed of a non-magnetic material.Austenitic stainless steel would often be quite suitable, but thematerial of the guide tube is dictated by the liquid within which itwill be disposed. The diameter of the guide tube will also be dependentupon the environment for the sensor, the length of the tube sensor, etc.Except as hereinafter mentioned, the guide tube may be of any suitablediameter such as from 1/2 inch to 2 inches OD. For applications having aconsiderable vertical extent, or turbulent liquid movement, largerdiameters would be preferred over smaller diameters.

The inside of the guide tube is sealed from the liquid 28. Onearrangement for accomplishing this purpose is shown in FIG. 1. The lowerend of the tube is sealed by the use of a neoprene or other suitableelastomeric plug 48 having a stem portion telescoped inside the lowerend of the tube and a flange portion overlying and abutting the loweredge of the tube. A bolt or the like 50 having a head 52 overlying theinner end of the plug and a shank extending downwardly through andbeyond the lower end of the plug and through a disc or the like 54 andthreaded into a nut 56, is utilized to squeeze the plug axially andthereby expand it radially to tightly grip the inner wall surface of theguide tube and the surface of the bolt to seal thereagainst and lock theplug in the tube. The portion 58 of the flange of the plug which extendsbeyond the outside diameter of the guide tube serves as a stop surfacefor supporting the lower edge of the float 34 when the float drops tothe lower end of the tube.

The junction box 38 may be closed at the top by a suitable cover 60 andthe circuit wires from the switches supported in the guide tube may beled out of the junction box through a side opening aperture 62 andthence to the devices to be actuated which may comprise visual or audioalarms, or relays suitable for operation of motor starters or solenoidvalves. In FIG. 1 the various circuit lines from the proximity switchesare shown as being gathered into a cable 64 leading to two pump controlrelays which control pump starters namely, pump 1 and pump 2 motorstarters as shown on the drawing. Pumps (not shown) may be connected tothe vessel or tank 20 containing its liquid 28 and their function may beto pump liquid out of the tank, or into the tank, as a result of theliquid level 30 rising or falling. Three proximity switches 66, 68 and70 are shown suspended in the guide tube. These switches are connectedtypically such that when the contacts of all switches are open, thepumps are inoperative.

In the FIG. 1 embodiment as a switch is closed, a signal is energized ora pump motor starter is energized to start the pump motor, perhaps topump liquid out of the tank. If, for example, liquid can enter the tank20 and the function of the sensor is to prevent it from exceeding theheight of switch 70, when the float reaches switch 68 such will closethe contacts of switch 68 and start pump number 1. If the liquidcontinues to rise and the float reaches switch 70, it closes thecontacts thereof and starts motor number 2. According to the invention,switches 66 and 68 remain closed as the rising float passes above them.Assuming that with both pumps in operation the liquid level within thetank is being reduced, and as the float descends and reaches each of theswitches 70, 68 and 66 the contacts thereof will be opened. Duringdescent of the float and opening of switches 70 and 68 the pumps maycontinue operative through the provision of the relays LH until switch66 is opened whereupon both pumps will be stopped. In any event, it willbe understood that the proximity switches 66, 68 and 70 are normallyopen switches (they are closed as the float rises) and as the floatreaches each switch it closes its normally open contacts and latchesthem closed as the float continues to rise beyond the switch. Suchcontacts remain latched closed until the float descends reaching andpassing each switch whereupon the contacts are unlatched and are openedand remain open until the float again re-ascends to effect a closingthereof.

The float 34 may be formed of any suitable non-magnetic material. Anysuitable plastic or austenitic stainless steel may be used. The floatshould be hermetically sealed. If desired, it may be filled with aclosed cell foam, so that in the event the float is punctured it willnevertheless retain its buoyancy. In the preferred form the floatcomprises a structure symmetrical along both longitudinal and transverseaxes and is generally egg shaped having a greater axial than transversedimension. The float has an outer wall 72 secured to and sealed at upperand lower edges 74 and 76 to a central float tube 78. The float tube 78is sized to be a free sliding fit about the guide tube. Inside the floatand secured to the float tube 78 substantially midway of its lengthviz., radially opposite its liquid line, is float magnet means 80. Suchfloat magnet means may comprise, as shown in FIGS. 2 and 12, a pluralityof small bar magnets 81 arranged around and extending radially withrespect to the guide tube axis and float tube axis, which areessentially coincident. The axes of such magnets also lie essentiallyperpendicular to the axes of the float tube and guide tube or in a plane98 disposed perpendicular thereto as shown in FIGS. 2A-2D. It will beobserved from FIGS. 2 and 12 that the magnet means 80 has north polesfacing radially inwardly and south poles facing radially outwardly.

The magnet means 80 may take various forms. For example, the magnetmeans may comprise a plurality of small bar magnets 81, as previouslymentioned, arranged in radial array with north poles facing radiallyinwardly and south poles outwardly. Such small bar magnets 81 may bepotted in a suitable potting material 83 such as an epoxy resin,melamine resin, urethanes, etc. shaped in the form of an annulus andsecured to the tube 78 in any suitable fashion. Alternatively, andpreferably, the magnet means 80 may comprise a strip of rubber-bondedbarium ferrite composite material sold under the trademark PLASTIFORM by3M Company. This is a flexible magnet containing magnetic particleswhose poles are arranged in uniform directions to create a north pole onone side of the strip and a south pole on the other side of the strip. Alength of such strip is shown at 82 in FIG. 11 wrapped around the floattube 78 with the ends cut to match as at 84. A suitable adhesive may beutilized to adhere the strip to the float tube. If desired, a suitablenon-magnetic contractive band (not shown) may be affixed around thestrip 82 to grip it to the float tube and insure against displacement.

Thus, with the embodiment shown in FIGS. 1 and 2 the north poles of thefloat magnet means face radially inwardly while the south poles faceradially outwardly. In addition, it will be noted that by positioningthe magnet means 80 midway of the vertical length of the float, and withthe float buoyancy such that the liquid level 30 is substantially midwayof the vertical dimension of the float, the magnet means and liquidlevel will be substantially horizontally coincident. Accordingly, theposition of the magnet means 80 corresponds quite accurately with theliquid level.

In the embodiment shown in FIGS. 1 and 2 the proximity switches 66, 68and 70 each comprises a conventional reed switch having a glass envelope87 (see FIG. 9) within which are positioned a pair of flexible magneticreeds or contacts of low magnetic permanence with one reed extendingfrom each end of the envelope and with the reeds having overlappingcontact faces substantially midway of the length of the envelope. Thesereed switches are readily available from a number of sources. They arearranged in the guide tube to have their axes extend longitudinallythereof. When a magnetic field of sufficient strength is impressedacross the contacts, it will cause them to close and when the magneticfield is removed the contacts will open. The contacts are physicallyspring biased by virtue of their construction so that they are normallyopen and are closed when exposed to a magnetic field of sufficientstrength and proper direction. Accordingly, the reed switches for use inthe embodiment of FIGS. 1 and 2 will close when subjected to a magneticfield of sufficient strength and will open upon removal of such field.Accordingly, they do not by themselves have any magnetic latchingcapability.

We have associated with each of these reed switches as shown in FIGS. 1and 2 small bias or latching magnets 86, 88 and 90, each of which hasits poles or magnetic axis arranged parallel to the axis of the reedswitch, namely, vertically. These three bias magnets may be formed ofany suitable magnet material but preferably are formed utilizing amaterial similar to the above mentioned magnetic strip but of a greaterthickness, such as 1/4 inch. A small piece of such material may be cutout and a hole pierced therethrough and the small piece then slippedover the upwardly projecting terminal 92 with the terminal extendingthrough the hole. Similar small pieces of such magnet material may beassociated with the upwardly extending terminal 94 and 96 at the upperend of each of the switches 68 and 70 respectively. Each bias magnet issized and positioned so that its field strength is insufficient alone toclose the normally open contacts of its associated reed switch but is ofsufficient strength to hold the contacts closed once they have beenclosed.

In FIGS. 1, 2 and 3 the bias magnets are shown in a position that isconsidered to be most desirable. However, the bias magnets may take ondifferent arrangements to achieve the same results. For example, thebias magnet may be placed at the end of the switch with its pole pointedradially upward, or in the middle of the switch with poles pointed upand down, parallel to the axis of the switch.

The effect of the bias magnets and the float magnet on the operation ofthe reed switches is best understood by reference to FIGS. 2A, 2B, 2Cand 2D. In these FIGS. the magnetic fields of the magnets areschematically represented by dashed lines. In FIG. 2A the float isassumed to be rising and its magnet means 80 creates a leading field 83extending upwardly and a trailing field 85 extending downwardly and inthe area where such fields sweep the reed switch 68 they are of oppositedirection. The polarity of the leading field of FIG. 2A results in thereed or contact 100 having a north polarity at its contact face. At thesame time reed or contact 102 will have a north polarity at its contactface as a result of the polarity of the bias magnet 88. With likepolarities at the contact faces of reeds 100 and 102 the contacts arerepelled and remain open as the float magnet 80 approaches the reedswitch.

However, when the float magnet is radially opposite the contact faces ofreeds 100 and 102, as shown in FIG. 2B, the contacts are closed. Thisresults from the trailing field 85 of the float magnet having adirection complimenting or augmenting the polarity of the bias magnetcreating a north-south polarity across the contacts to close them. Asthe float rises above and carries its magnet means 80 above the FIG. 2Bposition, the magnetic field of the bias magnet will continue to holdthe reed switch contacts in the closed position shown in FIG. 2B andalso as shown in FIG. 2C.

In FIG. 2C the float is descending such that the leading field is nowfield 85 below the float magnet and the trailing field 83 is above it.As the leading field reaches the reed switch it will at first augment orcompliment the bias magnet polarity across the reed switch maintainingthe contacts closed, and such is shown in FIG. 2C. However, when thefloat has descended slightly more, its trailing field, which is upwardlydirected will produce a north polarity at the contact of reed 100 whichwill thereupon be repelled from the north polarity of reed 102 asimposed by the bias magnet 88 and the switch will open and remain openas the float descends below the switch.

Consequently it can be seen from FIGS. 2A, 2B, 2C and 2D that byproviding leading and trailing fields in the guide tube of oppositedirection above and below the transverse axis of the float and byproviding a bias magnet in association with the reed switch ofsufficient strength to hold the contacts closed but of insufficientstrength to close them and wherein the strength of the float magnet andthe bias magnet are sufficient to close the contacts, the proximityswitches in the guide tube can be made to function as latching reedswitches.

It will also be observed that means are thus provided which will closeeach switch when the float passes by it moving upwardly and open eachswitch when the float passes by it moving downwardly. Means are alsoprovided for holding the switch closed independently of the distance thefloat moves above the switch. Further it will be noted that like polesof the bias magnets of FIGS. 1-3 point in the same directionlongitudinally of the guide tube and the downwardly facing poles of thebias magnets correspond in polarity to radially inwardly facing poles ofthe float magnet. This produces switches which are normally open whenthe float is at the bottom of the guide tube and close as the floatascends.

In FIG. 3 there is shown a modification of the embodiment of FIGS. 1 and2 wherein the polarity of the bias magnets has been reversed from thatshown in FIGS. 1 and 2. Corresponding reference numerals have been usedin FIG. 3 to indicate like parts. The bias magnets 86', 88' and 90',have their north poles facing upwardly and their south poles downwardly.The effect of this is to change each of the proximity switches fromnormally open to normally closed. Thus switch 66' which is normallyclosed, is opened and remains open as the float reaches and passes aboveit. Switch 68' is shown in its normally closed position just prior tobeing opened and switch 70' is shown in its normally closed condition.In other words the switch remains closed as long as the float magnet isbelow the switch and the switch will open and remain open as long as thefloat magnet is above the switch. Switch operation may also be reversedby reversing the poles of the float magnet. In other words, the switchesof FIG. 2 may be made normally closed by providing the float magnet withthe south poles pointing radially inwardly and the north polesoutwardly.

A substantial advantage arising from the constructions shown in FIGS.1-3 is that they permit the reed switches in a guide tube to be ofdifferent operating modes, viz., some of the switches may be normallyopen as in FIG. 2 and some of them may be normally closed as in FIG. 3;the mode of operation being obtained simply by the polarity orientationof the bias magnet associated with each switch. Another advantage of theembodiment of FIGS. 1-3 is that the float can be placed on the guidetube with either end up because the magnetic field created by the floatmagnets is symmetrical.

FIGS. 3A, 3B, 3C and 3D illustrate functioning of the normally closedreed switch of FIG. 3 as the float and its magnet means approaches andpasses the switch; FIGS. 3A and 3B showing the float as it is risingwhile FIGS. 3C and 3D as it is falling. In FIG. 3A the float magnetleading field reinforces the bias magnet field and the contacts 100'102' remain closed. As the float magnet reaches a position substantiallymidway of the length of the reed switch, the polarity of contact 100'will reverse so that it corresponds to the polarity of 102' and thecontacts will open. Having once opened, the strength of the bias magnetfield is insufficient alone to close the contacts, and they remain openas the float magnet passes above the switch. In FIG. 3C the float isshown descending and the leading field which was the trailing field asthe float ascended will, as before, tend to maintain the contacts in theopened condition until the float magnet has reached a pointapproximately midway of the length of the reed switch, at which time thepolarity of the lower contact 100' is opposite the polarity of uppercontact 102 and the contacts will close with the field forces beingsubstantially as represented in FIG. 3D.

With the construction shown in FIGS. 1 and 2 or that shown in FIG. 3 thebias magnet 88 or 88' can be located either above or below its switch.In either case switch operation is determined by the direction that thenorth and south poles point (up or down). Assuming that the floatmagnets have their north poles pointed radially inward, a bias magnetwith its north pole pointed downward will cause the switch to be closedwhen the float is above the switch and open when the float is below theswitch. If the north pole of the bias magnet is made to point upward theswitch operation is reversed.

We have found that a sensor constructed as disclosed hereinabove has afairly close sensitivity, viz., quite small vertical movements of thefloat will serve to close or open the switch. Thus by accuratelypositioning the switches in the guide tube, a sensor having considerableaccuracy is attained.

FIGS. 4 and 4A-4E show a somewhat different form of reed switch andfloat magnet combination in order to effect a latching operation. Inthis arrangement each of the reed switches has contacts made of amaterial capable of having a high residual magnetism. Such a reedswitch, referred to herein as a self-latching reed switch, has recentlybecome commercially available on the market, one being sold under thename MEMOREED FDR-8 manufactured by Fujitsu Limited of Japan. This typereed switch can be latched closed by a strong magnetic field andunlatched and opened by a weaker magnetic field of reversed polarity. InFIG. 4 there are shown three such switches 89, 91 and 93 arranged in theguide tube with the float magnet means being arranged to provide twofields of opposite polarity and different strengths for sweeping theguide tube.

As shown in FIG. 4 the float magnet comprises a plurality of magnets 110arranged so that the north poles point radially inwardly and their southpoles radially outwardly but with the axis of the magnets arranged in aconical configuration around the float tube 78' with the angle of theconical surface with respect to the vertical axis of the float tubepreferably being at about 30° . However, this angle may range from 15°to 60° . The float magnet means may be formed by a plurality ofindividual bar magnets potted or encapsulated in the aforesaid conicalarrangement within an annulus of any suitable potting or encapsulatingmaterial 112 which is thereafter slipped over the float tube 78' andadhesively secured thereto prior to final assembly of the float.Alternatively, the bar magnets may be potted in the potting material 112as an integral part of the float manufacture; the particular manner offabrication being for routine skill in the art.

According to another possible construction, the magnets 110 may beformed by arranging the elastomeric magnetic tape 82 in a conicalfashion rather than a cylindrical fashion as shown in FIG. 11 andsecuring the same to an annulus having a conical face. In either event,the magnetic field created by the conical arrangement of the floatmagnets should correspond to that schematically illustrated in FIG. 4Awhere the angle θ is equal to approximately 30° and represents theinclination of the axis of the magnets to the axis of the float tube orguide tube. It will be observed from FIG. 4A that there is a strongmagnetic field 110' directed upwardly and sweeping the axis of the guidetube, such axis being indicated by the letter B and a weaker magneticfield 111' of opposite direction extending downwardly and sweeping theaxis B. The stronger upwardly directed field becomes the leading fieldwhen the float is rising and the trailing field when the float isfalling and is the field existing within the cone defined by the magnets110.

In operation, assuming the float to be rising, reference to FIG. 4Bshows the contacts 114 and 116 in the closed condition with the strongmagnetic leading field 110' of the magnets 110 serving to maintain thecontacts in the closed condition. As the float rises and carries themagnets 110 past the contacts 114 and 116, the weaker trailing field111' will neutralize the residual magnetism in the contacts causing thecontacts to spring open. The contacts will remain in this condition asthe float continues to rise and will not close until it descends asshown in FIG. 4D. As the float descends, a point is reached when thefloat magnets are substantially radially opposite the contact faces, asin FIG. 4E, where the strong trailing field 110' causes the contacts toclose and they will remain closed as the float continues to descentbelow the reed switch.

Reverse operation of the reed switches may be obtained by reversing thefloat end-for-end on the guide tub such that the stronger field nowpoints downwardly and the weaker field points upwardly. It will also benoted that all of the reed switches in the FIG. 4 embodiment will havethe same mode of operation. In other words, this embodiment does notafford the capability of having some switches normally open and othernormally closed as with the embodiments of FIGS. 1-3. An advantage ofthe embodiment of FIG. 4 is that because bias magnets are not used inassociation with the reed switches, it is possible to place the reedswitches closer together vertically within the guide tube and thereforobtain more control functions in a given height.

Referring now to FIG. 6, a further modification is shown whichcorresponds closely to the FIG. 4 embodiment in that each of the reedswitches is of the self-latching type, namely, having contacts formed ofa material capable of relatively high residual magnetism. As theseswitches correspond to those of FIG. 4 they are correspondinglynumbered. The float magnet means of the FIG. 6 embodiment differssubstantially from that of FIG. 4 but the effect is to create oppositelydirected magnetic fields of differing strengths similar to the fieldscreated by the conical magnet arrangement of FIG. 4. In accomplishingthis, a radially extending arrangement 120 of magnets having inwardlyfacing north poles and outwardly facing south poles are disposed justabove a radially extending array of magnets 122 having radially inwardlyfacing south poles and outwardly facing north poles. The magnet array120 has larger and stronger magnets than the array 122 and consequentlyfields of different strengths above and below the magnet assembly areformed. An alternate but similar arrangement may utilize a washer ofmagnetic material such as mild steel to replace magnet array 122. Suchmagnet means may be formed by potting or encapsulating small bar magnetswhose polarity is arranged as shown, or by utilizing the rubber-likemagnetic material mentioned in connection with the FIGS. 1-3embodiments.

It will be apparent that with a stronger magnetic field extendingupwardly and a weaker magnetic field of reverse direction extendingdownwardly, the effect on the reed switches will be similar to thatexplained in connection with FIGS. 4B-4E, namely, the reed switches arenormally closed and are opened and remain open as the float rises andare closed and remain closed as the float falls.

Referring now to FIG. 5, a further embodiment of the invention is shownutilizing self-latching reed switches of the kind shown in FIGS. 4 and 6and a float of the kind shown in FIGS. 1-3. In order to provide aworkable arrangement in the FIG. 5 embodiment utilizing this type ofself-latching reed switch and float magnet means, each of the reedswitches is provided at one end with a magnetic shield 136 of lowresidual magnetism as shown in FIGS. 5A through 5D. The shield iscylindrical to conform to the outside surface of the reed switchenvelope and extends over approximately forty percent of the length ofthe envelope substantially as shown in FIGS. 5A-5D. As a result, theswitch is opened and remains open when the float passes over the switchin a direction that carries the float magnet over the shielded end last.

Referring to FIGS. 5A through 5D, the magnetic shield covers the lowerend of the switch. With the float magnet 80 slightly above a mid-pointof the reed switch, as shown in FIG. 5A, the switch contacts arestrongly influenced by the leading field 140 and are held closed. Whenthe float descends to the level of the mid-point of the contacts, asshown in FIG. 5B, the upper reed 116 is influenced by the trailing field142 which develops a south polarity in the upper reed contact and theleading field 140 produces a south polarity in the lower reed 114 sothat the contacts repel each other to open the switch. This condition isshown in FIG. 5B.

As the float continues to descend the lower reed 114 is shielded fromthe trailing field 142 so that the switch remains open as in FIG. 5Cthough the float continues to descend substantially below or beyond theswitch. The contacts will remain open until the float ascends again suchthat the field 140, at that time the trailing field, can influence thecontacts and produce a north and south polarity across them such thatthey are closed as shown in FIG. 5D. To obtain reverse operation of theswitch with this embodiment, the shield 136 instead of being placed atthe lower end of the reed switch is placed at the top end thereof.

An advantage of the embodiment shown in FIGS. 5, 5A-5D is that the floatcan be placed on the guide tube with either end up and the reed switcheswithin the guide tube may have different operating modes, i.e. some ofthe switches may be normally opened and the others normally closed. Inaddition, it is possible to place the reed switches in closer verticalproximity in view of the fact that a bias magnet is not utilized.

In FIG. 7 a further embodiment is shown. Here the float magnet meanscreates oppositely directed fields which extend upwardly and downwardlyin the guide tube 32 to create leading and trailing fields of oppositedirection, but instead of arranging magnets with common poles facingradially inwardly and the opposite poles facing radially outwardly, thepoles are arranged to face parallel to the axis of the guide tube.

As shown, small bar magnets 85 are arranged circumaxially of the floattube in a first array. Four such magnets may be provided at equalangular positions. Their north poles may face downward. Beneath andadjacent them is a second array of four more small magnets 87 of equalsize with their north poles facing upward. As shown by the flux lines inFIG. 7, this arrangement creates oppositely directed force fields withinthe guide tube for sweeping the reed switches, and with reed switches ofthe kind and arrangement of FIG. 2, will result in the same action asdescribed in connection with FIG. 2. By making the magnets 87 of thelower array smaller, such as half the size of magnets 85, a less intensedownwardly directed field is created and when associated withself-latching reed switches as shown in the FIG. 4 embodiment, willfunction to operate such switches similar to the FIG. 4 arrangement.

In FIG. 8 a still further modification of the invention is disclosed. Inthis modification the construction is similar to that of FIGS. 1 and 2except as mentioned. The float 34' has a float tube 78' formed ofmagnetic material having the capability of little or no residualmagnetism. This magnetic float tube acts as a field conductor for themagnetic field created by each of the bias magnets 88. The action issuch that as the float rises it causes the reed switch contacts to openas the lower trailing edge of the float passes the switch. This occurswhen the float is rising because when the lower edge of tube 78' justclears the vertical mid-point of the reeds, the tube at that timeeffectively reduces the magnetic flux through the lower reed such thatthe magnetic attraction between the reeds cannot overcome their physicaltendency to open and accordingly they open. On the other hand as thefloat descends, though the bias magnet is not of sufficient strengthwithout the presence of the magnetic tube 78' to close the contacts, theconductive magnetic effect of the tube 78' so reinforces the magneticflux across the contacts that they close. While inverting the biasmagnets so that their poles point in the opposite direction will notalter the mode of operation, i.e. make the switch normally closed ratherthan normally open, removing the bias magnet to the opposite end of thereed switch will effect a reversal in mode of operation, namely, toalter the switch from, for example, a normally opened to a normallyclosed switch.

In FIGS. 9 and 10 we have shown a reed switch subassembly andarrangement for suspending it in the guide tube. We have found that thereed switches may be conveniently supported in the guide tube by theconductive wires themselves. A common lead or conductor 200, which maybe bare of insulation, extends down through the guide tube past eachswitch as shown in FIG. 1. Each reed switch, such as reed switch 68 inFIG. 9 has its bias magnet 88 attached to the upper terminal 93 of theswitch. An electric conductor in the form of a spring connector 202 isconnected to the terminal 93 by a splice 204. The connector 202 isformed of an uninsulated spring wire and its upper end is curled into apair of loops 206 and 208 which are adapted to be resistingly separatedand sprung over to grip opposite sides of the conductor 200, and as thewire 202 is bare, the loops will make an electric contact with conductor200 as well as serving to suspend the reed switch from the conductor200.

The lower end of the reed switch has a terminal 210 and a splice 212serves to connect the terminal to an insulated conductor wire 214 whichextends upwardly alongside the reed switch and through the loops 206 and208 where by a butt splice 216 it is connected to a lead wire 218. Thereed switch, bias magnet and splices 204 and 212 are received within aprotective and insulating sleeve of non-electrically conductive,non-magnetic material 220, which is filled with a potting resin such asepoxy resin. Thus a sub-assembly is constructed which may be connectedto the conductor 200 at any convenient location thereby facilitatingassembly of the sensors. It is apparent that vertical adjustmentsbetween the reed switches may be readily affected by simply sliding thegripping loops 206 and 208 up and down the conductive wire 200. Byhaving the lead wire 212 pass through the spring connector loops theswitch sub-assembly is secured to the conductor 200 so that is will notbe unintentionally removed nor can it be shaken or vibrated therefrom,the same acting as a lock.

The conductor 200 is brought out of the upper end of the guide tube 32and at that point may be simply folded over such end as at 222 therebyto retain the string of reed switches and conductors at any desiredlevel. If desired the conductor 200 may be provided with suitablemarkings indicating the depth of the reed switches below the upper endof the guide tube. Switch level, and consequently level sensingadjustments, may be made merely by removing the string of switchassemblies from the guide tube and resetting the levels of each switch.

We have found that the bias magnets 86, 88 and 90 of FIGS. 1 and 2 andcorresponding bias magnets of other embodiments, may be turned up to 90°so that their axes extend radially with respect to the reed switchterminal, and the sensor will work satisfactorily. In this case the modeof switch operation will be similar to that previously describedprovided the radially inwardly facing pole of the bias magnetcorresponds to the downwardly facing pole. For example, in the case ofFIG. 2, if the bias magnet is arranged so that its north pole facesradially inwardly rather than facing downwardly, the switch willfunction just as previously described, and the same is true of the otherembodiments utilizing the bias magnets.

What is claimed is:
 1. A liquid level sensor comprising, incombination:a guide tube for vertical positioning in the liquid whoselevel is to be sensed; a float externally surrounding the guide tube forlongitudinal movement thereon to rise and fall with the liquid level andhaving a portion of magnetic material of low residual magnetismencircling the guide tube; a reed switch in the guide tube having reedsof low residual magnetism extending substantially parallel to the axisof the guide tube; a bias magnet in the guide tube adjacent one end ofthe reed switch with the magnet poles facing in opposite directionslongitudinally of the guide tube; and said portion of magnetic materialon the float functioning as a field conductor for the magnetic field ofthe bias magnet causing the switch reeds to open and remain open as thefloat passes by them moving in one direction, and close and remainclosed as the float passes them moving in the opposite direction.
 2. Theinvention defined by claim 1 wherein there are a plurality of said reedswitches each disposed as aforesaid and arranged in vertically spacedapart relation, and there is a bias magnet for each reed switch disposedas aforesaid.
 3. The invention defined by claim 2 wherein the biasmagnets are disposed adjacent the upper end of each reed switch.
 4. Theinvention defined by claim 2 wherein the bias magnets are disposedadjacent the lower end of each reed switch.
 5. The invention defined byclaim 2 wherein at least one bias magnet is disposed adjacent the upperend of one reed switch and another bias magnet is disposed adjacent thelower end of another reed switch.
 6. The invention defined by claim 1wherein said portion of magnetic material comprises a sleeve secured tothe float and closely encircles the guide tube.
 7. The invention definedby claim 6 wherein said sleeve is adapted to engage the guide tube andserve as a bearing surface during movement of the float on the tube.